Memory is but one type of integrated circuitry. Some memory circuitry allows for both on-demand data storage and data retrieval. For example, memories which allow both writing and reading, and whose memory cells can be accessed in a random order independent of physical location, are referred to as random-access memories (RAM). Read-only memories (ROMs) are those in which only the read operation can be performed rapidly. Entering data into a read-only memory is typically referred to as programming, and the operation is considerably slower than the writing operation utilized in random-access memory. With random-access memory, information is typically stored with respect to each memory cell either through charging of a capacitor or the setting of a state of a bi-stable flip-flop circuit. With either, the stored information is destroyed when power is interrupted. Read-only memories are typically non-volatile, with the data being entered during manufacturing or subsequently during programming.
Some read-only memory devices can be erased as well as written to by a programmer. Erasable read-only memory typically depends on the long-term retention of electronic charge as the information storage mechanism. The charge is typically stored on a floating semiconductive gate, such as polysilicon. One type of read-only memory comprises FLASH memory. Such memory can be selectively erased rapidly through the use of an electrical erase signal.
A FLASH memory cell typically comprises a single floating gate transistor. For multiple storage cells, such as used in large semiconductor memories, the storage cells of the memory are arranged in an array consisting of rows and columns. The rows are typically considered as comprising individual conductive gate lines formed as a series of spaced floating gates received along a single conductive line. Source and drain regions of the cells are formed relative to active area of a semiconductor substrate, with the active areas being generally formed in lines running substantially perpendicular to the lines of floating gates. The sources and drains are formed on opposing sides of the lines of floating gates within the active area with respect to each floating gate of the array. Thus, lines (rows) of programmable transistors are formed.
Electrical connections are made with respect to each drain to enable separate accessing of each memory cell. Such interconnections are arranged in lines comprising the columns of the array. The sources in FLASH memory, however, are typically all interconnected and provided at one potential, for example ground, throughout the array. Accordingly, the source regions along a given line of floating gates are typically all provided to interconnect within the substrate in a line running parallel and immediately adjacent the line of floating gates. These regions of continuously running source area are typically interconnected outside of the array, and strapped to a suitable connection for providing the desired potential relative to all the sources within the array.
A common manner of making electrical connections with respect to each drain typically involves deposition of an insulative layer, for example borophosphosilicate glass (BPSG). Individual contact openings are etched therethrough to the drains, typically utilizing photolithographic patterning and masking techniques using photoresist. One prior art technique of filling such contact openings comprises forming a composite of polysilicon, then titanium, then titanium nitride, and finally elemental tungsten. The individual contact plugs so formed typically ultimately connect with aluminum-containing conductive lines formed elevationally outward of the insulative layer. Unfortunately, and typically with contact openings at least 5,000 Angstroms deep and having minimum aspect ratios of at least 7:1, an open seam, crack, or synonymously a crevice, can form within the deposited polysilicon and which can extend to deep within the contact openings. The polysilicon is typically recessed back to within the contact openings, and whereupon the elemental titanium, then titanium nitride and tungsten are deposited to within the openings. This thereby provides a higher conductive interface between a primary polysilicon plugging material and an overlying aluminum line to be fabricated, and desirably separates the polysilicon from the aluminum.
The polysilicon is typically first etched back by chemical mechanical polishing (CMP) or other technique. The presence of crevices tends to result in greater removal of the polysilicon from centrally within the contact openings between the contact opening sidewalls than occurs more proximate the contact opening sidewalls. Thereby in some instances, polysilicon still remains at the upper portion of the contact opening proximate the upper surface of the insulative material within which the contact openings are formed. Further, the subsequently deposited titanium, titanium nitride and tungsten might not completely cover or fill the crevices and, regardless, can result in some polysilicon, at least at the edges of the contact openings, coming into contact with aluminum of the lines which are subsequently formed. This is highly undesirable, and can lead to failed circuitry.
The invention was motivated in overcoming the above-described issues and problems associated with FLASH memory circuitry. However, the invention is in no way so limited in certain aspects, and has applicability to any method of forming a polysilicon-comprising plug as herein disclosed and claimed. Accordingly, the invention is only limited by the accompanying claims as literally worded, without interpretative or other limiting reference to the specification, and in accordance with the doctrine of equivalents.